During last years monoclonal antibodies (MAB) directed against vascular endothelium markers demonstrated their efficiency for visualization and targeted delivery of therapeutic drugs to tumors. Endoglin (CD105) which serves as a key element that determines endothelial cells quiescence or activation is one of such markers. Endoglin is highly expressed on the vascular endothelium of growing tumors. A first panel of MAB against endoglin in our country was produced at the hybridoma technology laboratory of RRC RST named after A.M. Granov. On the basis of these MAB ELISA was created allowing detection of endoglin in human plasma and other biological fluids. Several MAB had been shown to bind endoglin on the membrane of the cultured endothelial cells and to persist there for several hours. During the first 30 min after binding some of the immune complexes "endoglin-MAB" were internalized into the cytoplasm and were found included in the endosomes. In future these MAB can be used to create the reagents for the addressed delivery of isotope tags both on the membrane and into the cytoplasm of endothelial cells.